Hilborn stack idle problems

[PSweeney]

Running a Holborn stack set up on a 420 SBC. With the butterfly's closed to .002 thou and bv set to 20% it idles at 2500rpm in neutral and 1800 to 2000 in gear, any richer on the bv and its noticeably rich and milking oil. Timing is at 32 degrees all in. How can we get this thing to idle lower, it's pretty undefinable at low speed and getting through a lot of fuel. Can the holes in the throttle bodies above the butterfly's be tuned with bleeds to bring the idle down ?

[Spud Miller]

 Sounds like you have an air leak. The intake gasket, in the injector...something like that. You didn't mention how big the butterflies are, but a motor that size shouldn't have a problem with the aspiration vents in the nozzles being too much air.

 Maybe your intake gasket shifted as you put the injector on and it's sucking air from the lifter valley.

 Is this the model Hilborn that has the banks of butterflies that bolt to a base? Maybe one of those is loose or not sealing.

 You could check the exhaust temps with a heat gun and see if you can narrow down the problem to one side of the motor, or a pair of cylinders or something like that. It may help you track it down.

 Spud

[dreracecar]

Making the BV richer will not increase idle, more air or less fuel will raise idle rpm. Trans in gear puts a load on the engine and brings rpm down and is why you run a secondary idle bypass to lean the fuel system and bring the rpm's up.

[PSweeney]

thanks guys.  Will look at a manifold leak.  We we're getting some oil out the left bank header but assumed that was due to the damaged valve stem seals we had to swap out.  It could also be a leak from the valley I guess.

[PSweeney]

Just re-visiting this issue.

Firing the car at home, the idle in neutral was still 2500rpm, blip the throttle and it doesn't come back down on it's own and will sit at 2800rpm until you drop back into gear.  We looked for vacuum leaks, resealed the manifold, split the throttle bodies and used new gaskets.  Fired the motor and it's no different, 2500rpm in neutral, around 1800 -2000rpm in gear.

We ran the car again this weekend and managed to get 5 passes on it.  The converter is loose enough that it can be driven at low speed and staged but I just don't like it that high.  The BV is already set at 25% and is milking oil on each run.

This is a Hilborn 2 1/8" stack system on a 421cu in SBC, iron heads, 32 degrees timing locked out.  Running the quarter it ran 9.005 @ 148mph which is the best ET and MPH on the engine from when it last ran in an altered. 

I'm launching off idle, it revs well, no stumbling, pulls hard and stays lit in the shutdown, the xxxx thing will just not idle below 1800rpm.  Where do I go next ?

[ricardo1967]

I'd disconnect the spark plug cables, one by one, trying to identify if there's one cylinder contributing more than the others for this elevated low idle. That could help, for example, to catch a vacuum leak between intake manifold and cylinder heads.

[32bantam]

Are the butterflys square in the hole.   Back off on blade shaft adjusters...loosen every blade and tap lightly with a small hammer handle or plastic screwdriver handle to seat the blade. you will hear it seat....do them all ....re tap each and tighten.
Is the idle bypass stuck???

[PSweeney]

I'd disconnect the spark plug cables, one by one, trying to identify if there's one cylinder contributing more than the others for this elevated low idle. That could help, for example, to catch a vacuum leak between intake manifold and cylinder heads.

thanks will try that

[PSweeney]

Are the butterflys square in the hole.   Back off on blade shaft adjusters...loosen every blade and tap lightly with a small hammer handle or plastic screwdriver handle to seat the blade. you will hear it seat....do them all ....re tap each and tighten.
Is the idle bypass stuck???

new butterflys set at 0.002", can't see /feel any visible wear in the bores.  Slight wear in the front of the throttle shaft where the cable connects so we're looking at bushing it.  Also considering getting a sycrometer to check how even they are.

This is a stack system, so in terms of plumbing at the pump we have inlet in, primary bypass, shut off return, main line to BV and secondary bypass off the BV.  If by idle bypass you mean the secondary bypass off BV, I'm led to believe this is blocked at anything less than 25 degree throttle opening ?, so should be blocked at idle.  In terms of it's operation, coming off the burnout and run, there is no surge or stumble so we assume it's opening at part throttle to reduce the fuel pressure on throttle closure.

[dreracecar]

If its an older unit, you will find that the steel shaft is worn more than the alu housing is. Refurbishing is needle bearings in the bores and new shafts, You need some slop in there, to persision and it can bind up

[BK]

 Secondary bypass is OPEN from idle to about 20 deg. of opening. You primary bypass and idle will generally be the same. Containing a spring loaded check for idle pressure and your main bypass jet or pill. The secondary should by a spring check set significantly higher than your idle check maybe 25 lbs or so.

[retroboy]

I've come across this high idle but otherwise runs fine before and it was simply the popett and spring weren't sitting correctly in the pill holder. Drop in type pills.                   Cheers

[PSweeney]

so we've put a few more runs on the cars, changing oil every run.  The idle is still high and we popped the idle bypass, Which had the poppet, spring and 3 shims.  We took 2 shims out and saw no difference in the idle or oil milking.  BV is still at 25% or so.  Is there a recommended ratio between BV and Idle leakdown % ?

[Spud Miller]

 For methanol, I always use 7% of the engine size for an initial leakdown number. That's assuming a 1-2 PSI main check valve.

 So, for you:  420 x .07 = 29

 29% should be really close...within a flat or three...tweak from there for idle EGT of 350-450F and good throttle response when you whack it from an idle.

 Do you have any idle temperature information? Hand-held temp gun shooting the pipes?

 Spud

[slingshot383]

Running that low of timing in a SBC, I bet your main is fat.  That motor would be happier in the 36* - 40* of timing and leaned out main.